Japanese Unexamined Patent Application Publication No. 2007-89262 discloses power conversion equipment used typically in hybrid vehicles. The disclosed power conversion equipment includes a first DC power supply, a driving circuit, an electric power converter, a second DC power supply, and a control means.
The first DC power supply includes a battery, for example. The driving circuit includes an inverter, fed with a DC voltage, that conducts electric power conversions for controlling the motor drive for driving the vehicle. The electric power converter is connected between the first DC power supply and the driving circuit and formed of a boost converter that conducts a DC voltage conversion between the first DC power supply and the driving circuit. The second DC power supply includes capacitor C0 connected in parallel to the electric power converter via a switching means. The control means sets the reference value of the output voltage that the electric power converter feeds to the connection point of the driving circuit and the second DC power supply and controls the operations of the electric power converter to make the output voltage and the set reference value thereof coincide with each other.
The control means includes a first reference value setting means that sets the voltage corresponding to the required output of a motor as a reference voltage, a second reference value setting means that sets a voltage higher than the voltage corresponding to the required output and capable of charging the second DC power supply as the reference voltage, and a selecting means that selects any of the first reference value setting means and the second reference value setting means in response to the request of the driver of the vehicle.
The relations between the motor torque Tm, the output voltage V of the inverter that configures the driving circuit, the motor current Im, and the number of motor revolutions (hereinafter referred to as the “motor speed”) Nm in the conventional power conversion equipment disclosed in Japanese Unexamined Patent Application Publication No. 2007-89262 are described in FIG. 10.
As described in FIG. 10, the motor torque Tm and the motor current Im are set to be almost constant by the V/F control in the range, in which the motor speed Nm changes from 0 to the predetermined revolution number N0, and the output voltage V of the inverter increases in response to the increase of the motor speed Nm. Since the voltage that the inverter can feed is limited to the maximum voltage Vmax, it is necessary to conduct a flux-weakening control of the motor for further increasing the revolution speed of the motor, as the motor speed Nm exceeds the predetermined revolution number N0 to the larger side. If the flux-weakening control is conducted, the value Im of the motor current that flows through the motor will also increase, as described in FIG. 10, in association with the increase of the reactive power component.
If one wants to drive the motor at a higher speed using an inverter but not conducting any flux-weakening control, it will be necessary to increase the converter capacity of the inverter. In other words, a tradeoff relation exists between the converter capacity of an inverter and the maximum revolution speed of a motor. If one wants to construct a system that facilitates driving a motor at a higher speed without conducting any flux-weakening control, the manufacturing costs of the electric power converter including the costs of increasing the converter capacity will soar.
The charging capacity of a capacitor is expressed by the following formula in the case in which the capacitor constituting the second DC power supply is charged by the battery voltage boosted by a boost converter.The charging capacity of the capacitor=the charging current×the charging time
In other words, the charging current (the converter capacity of the boost converter) and the charging time are inversely proportional to each other. If one wants to shorten the charging time, it will be necessary to increase the charging current. If the charging current is increased, the converter capacity of the boost converter should be large and the costs for increasing the converter capacity will soar.
In view of the foregoing, it would be desirable to obviate the problems described above. It would be also desirable to provide power conversion equipment that facilitates driving a motor at a high revolution speed and/or charging a DC power supply at a high speed with low costs.